This invention relates to the field of metal joining, and more particularly, to a trackless movement apparatus and full-penetration method for welding of a butt joint of large steel structures.
Arc welding technologies are widely used in modern manufacturing and industrial processes. However, in practical applications, conventional methods of arc welding have presented following drawbacks. First, in most case, multi-pass welding and beveling of the plate edge is used to complete a butt joint of thick steel plates of large structures, which increases welding time and cost. Second, mechanized equipment for welding of large steel structures is more complicated and expensive. Third, conventional automatic arc welding equipment normally employs a track for making a long weld, hence seam tracking plays a critical role in generating sound welds in many automatic welding applications. The seam tracking technique, however, cannot be widely used in welding production currently due to its low reliability and high cost.
To perform full penetration welding, it is normal practice to make butt welds from both sides of the plates. However, in large steel construction the ideal is to complete butt welds from one side, for example, it is difficult to turn a ship over for welding the second side. This procedure has led to the development of welding technologies of full penetration weld from one side. As widely known, full penetration weld from one side for a butt joint could be typically achieved in two ways.
First, methods have been proposed to sense joint penetration, such as infrared sensing, ultrasound, and pool oscillation primarily for gas tungsten arc welding process or measuring keyhole to control weld penetration for plasma arc welding process. However, these sensing technologies for welding penetration have not being used in welding production because of their sensitivity for welding conditions. For example, when weld joint gap is not constant in an entire weld line sense and control of weld penetration will be very difficult for an automated arc welding process.
In the second method for full penetration welding, a weld backing made from copper, with its length identical to the length of the weld line to be welded and its configuration fitting to the shape of the weld line, is used to hold against the underside of the weld zone where a groove is formed. In this condition, an arc welding torch or welder is moved along the weld line to weld throughout the entire length of the weld line. After completion of welding, the weld backing and its supporting members are removed.
In another application of weld backings, a number of ceramic weld backings in adequate shapes with adhesive aluminum foil adhere to the underside of the groove face of the weld zone along the entire weld line. Then, an arc welder is moved to weld throughout the entire length of the weld line, and after completion of welding, the ceramic weld backings are removed.
There are, however, a number of flaws with these conventional weld backings. For the weld backing made from copper, the length of the weld backing should be the same as the length of the weld line and the weld backing having the same curvature as that of the weld line is required in the case of a curve of weld line. Therefore, general-purpose ability of such a weld backing is poor for straight or curve weld lines and the cost of the material, processing and application increases when the weld line becomes longer. In addition, the installation and removal of such weld backings in practical welding production is difficult.
Another attempts have been proposed to overcome these drawbacks. For example, U. S. Pat. No. 5,590,829 describes a mobile weld backing mounted on a wheeled carriage to place at the underside of the workpiece during welding and to provide a moveable weld backing system along a weld line to perform full penetration welding of a box-shaped workpiece. By mounting a backing on a wheeled carriage, the foregoing flaws can be improved to some extent. However, the prior art has still some drawbacks in practical applications. In the invention, because the welder in top side of a weld line and the wheeled carriage mounting a weld backing in the underside separately travel along an entire weld line the control of the movement of the welder and the wheeled carriage with a backing synchronously is complicated. Another disadvantage of the invention is poor in general-purpose application because the method can be only used in welding of box-shaped workpiece.
Consequently, the obvious expense and complicated equipment and operation of the conventional arc welding processes have been forcing fabricators to explore new technologies for possible cost savings, increase of productivity, and simplification of machine and operation in welding of large steel structures.
Therefore, in order to overcome the above-described flaws accompanying conventional arc welding technologies, it is the main object of the invention to provide a trackless movement and full-penetration apparatus and high-efficiency arc welding methods based on this apparatus.
Accordingly, a general object of this present invention is to provide a trackless movement and full-penetration arc welding apparatus and welding methods based on this apparatus to improve productivity of conventional arc welding technologies and simplify equipment for fabricating large steel structures. In this invention, all of the above-described difficulties accompanying conventional arc welding processes can be overcome.
The basic idea of this present invention is that an apparatus with the trackless movement and full penetration functions will be assembled with a butt joint based on a preexisting gap (3 mm-10 mm) of a weld line. In the apparatus, a movable weld backing is held against the underside of the weld line to support liquid metal and control backside bead to achieve full-penetration welding of a butt weld. The movable weld backing and a fixing plate of the welding torch are arranged the underside and the top sided of the workpiece, respectively, using a connecting member through the pre-existing gap of the weld line. Thus, the moveable weld backing moves synchronously with the movement of the welding torch along an entire weld line to continuously perform full penetration welding. Three compression springs are installed a guide wheel unit and two roller units, respectively, to hold the movable backing against the underside of the workpiece in flat position for welding. The movable backing made from copper is of water cooling type. In the weld backing, shielding gas, such as CO2, as backing purge gas is used to protect the backside weld region from atmosphere contamination (oxygen and nitrogen) for welding of alloy steel materials.
In order to perform trackless movement, the pre-existing gap of the weld line is used to act a xe2x80x9cguide slotxe2x80x9d and a guide wheel unit to track the gap is designed in this apparatus. In practical applications, the guide wheel unit is placed ahead of the welding torch and a guide metal wheel with the inclined planes contacts with the pre-existing gap to guide the welding torch and the apparatus to move along the weld line to automatically perform seam tracking. For automatic arc welding systems, the invention of through-the-pre-existing-gap xe2x80x9cguide slotxe2x80x9d and the guide wheel unit can save an external track and a seam-tracking device, and increase the accuracy and reliability to track a weld line. The apparatus substantially simplify conventional automated arc welding equipment and methods, and welding operation, thereby, in the practical applications an electrical or a manual winch can be used as a driver to drag the apparatus with a welding torch to travel along a weld line to perform full penetration welding, irrespective of the length and configuration of a butt joint. In the automatic or semi-automatic (for a handle winch) arc welding system, the winch may be temporarily placed to an end of the base metals to be butt-welded and its chain is connected to the apparatus to drag it to move along the weld line with the winch rotation. During welding the winch is stationary and does not move with the welding apparatus. This means the welding system cannot be limited by an external track to weld a long butt joint after setting up the welding apparatus.
Another applicable trackless mechanized carriage with the moveable weld backing is designed. In the trackless mechanized carriage, an electric or manual winch or a servomotor can be use to drive the mechanized carriage apparatus with the moveable weld backing and the torch to perform full penetration welding.
Therefore, the movable weld backing for full penetration welding and the guide wheel unit and through-the-pre-existing gap xe2x80x9cguide slotxe2x80x9d for seam tracking is integrated in the apparatus of this present invention.
According to this apparatus, a single-pass and one-side full penetration welding method can be performed by use of a plasma arc to melt weld zone and weld metal to fill the pre-exiting gap and form the weld reinforcement using the movable backing. Plasma arc is chosen preferably in the welding process because of its higher concentrated heat and higher temperature in contrast of non-constricted arc. Thus, plasma arc can deeply penetrate metal materials and weld the thick plates in some practical applications. Additionally, in this process, the flux-cored wire as a filler metal can be added to the leading edge of the weld puddle using a mechanized wire feeder. Based on the physical and chemical characteristics of the flux-cored wire, it as a filler metal for trackless movement and full penetration plasma arc welding process can bring the following important effects:
(a) A layer of liquid slag between the moveable backing and the liquid metal of the backside weld pool will act as a xe2x80x9clubricantxe2x80x9d to increase the sliding ability of the moveable backing. Their interfacial surface tensions, the slag-liquid metal and the slag-solid backing, can be altered by using of changing or adjusting the components in the flux-cored wire so that the liquid slag of the flux-cored wire can act as a xe2x80x9clubricantxe2x80x9d effect in this process.
(b) The layer of the liquid slag acts as a very efficient heat xe2x80x9cinsulatorxe2x80x9d, and thus reduces the rapid dissipation of the heat of arc to protect the backing from the high temperature of plasma arc.
(c) The liquid slag film can provide a shielding layer for the weld pool from atmosphere contamination (oxygen and nitrogen).
(d) Substantially improve the composition and microstructure of the weld and mechanical properties of the weld joint due to increase of alloy element transfer efficiency and deoxidations.
Therefore, the new arc welding process, flux-cored wire plasma arc welding based on the apparatus of this present invention, can implement full penetration welding in single-pass without a preparation of the bevels for a large steel structure.
Based on the apparatus of the present invention, three arc welding methods, flux-cored wire plasma arc, submerged arc, and flux cored arc welding processes can be able to perform full penetration welding of a butt joint for large structures from one side.
According to the present invention, the apparatus and methods are applicable not only to weld a butt joint but also to weld a corner joint by use of a fitting moveable weld backing and practical arrangement of the mobile apparatus of the present invention.